Rack-based enclosures are commonly used for supporting groups of electronic assemblies such as cards, i.e., components mounted on printed circuit boards, forming computer systems or groups of computer components. The racks typically provide an environment that enables heat removal and protection from excessive heat, shock, vibration and/or dust. The electronic assemblies are used to form electronic applications, such as server farms that serve the networking needs of large organizations.
In response to demands for high performance systems, components are being designed with increased cooling requirements, printed circuit boards are being designed with increased component densities, and/or cards are being mounted closer together (i.e., positioned in more-closely spaced parallel planes) within computer chasses. While these factors all tend to increase cooling capacity requirements, a number of factors are decreasing typical cooling capabilities. For example, an increasingly close card proximity tends to increase airflow impedance, and decrease the allowable maximum component height on each card. Furthermore, high-power, high-speed circuits sometimes require very small or indirect ventilation openings so as to contain electromagnetic radiation. These problems tend to create cooling issues such as hot spots, dead zones and/or insufficient cooling capacity.
Present day rack-based enclosures commonly comprise a framework that carries and supports cards in a plurality of vertically stacked tiers. Typical industry standards for such enclosures allow for structures having a height of 40U or 42U, where U is 1.75 inches, and a useful internal width of approximately 17.7 inches with an external width of approximately 24 inches. Some typical configurations for such structures include a number of relatively slim tiers (e.g., about 1U in height), and others include a smaller number of taller tiers (e.g., 6 or 7 U in height) that hold a plurality of blades (i.e., printed circuit board cards oriented with their plane normal to a horizontal axis). In this latter configuration, the blades are typically stacked laterally across the width of the tier (i.e., the cards share the same approximate footprint in parallel planes that are normal to a width direction of the enclosure).
Each such card might be edge-mounted to a card connector on a bottom wall or back-plane (back wall) of the tier. In either case, the tier will typically include standard-configuration cards that mount such that their depth dimension (with respect to the tier) is greater than their height dimension, thus minimizing the height of each tier. In some cases, a single tier can include two such lateral stacks, one in a front half of the tier and one in a back half of the tier, with a ventilated mid-plane (i.e., having ventilation holes) dividing the front and back halves and serving as the rearward wall for card insertion, and carrying card connectors for both halves.
To maintain component temperatures inside rack-based enclosures, each tier of the enclosure typically includes ventilation holes in front and rear panels of the tier. A cooling fan is configured to draw ambient air through the enclosure, with the air traveling laterally across a depth direction (i.e., in either a front-to-back, or a back-to-front direction), the ventilation holes providing an entrance and an exit. Optionally, chilled air could be directed from an air cooling unit into the ventilation holes used for air intake.
Within each tier, the air flows unidirectionally through the depth dimension of the tier, tangential to the cards, typically at speeds of 1 m/s to 2 m/s, and possibly at speeds approaching 4 m/s. This tends to cause multiple components, and possibly numerous components, to be cooled in series. Consequently, the downstream components are cooled by preheated air, and thus are cooled by lesser amounts than are the upstream components. Furthermore, for tiers having front and back lateral stacks of cards, one lateral stack will be cooled by air that was preheated by the other stack.
An exemplary system might move air from front to back across two lateral stacks of cards. These cards might include single card processor units and I/O cards, each being mounted to a perforated backplane. The system air movers (e.g., fans) provide mass flow for a given temperature rise, e.g., 15 degrees centigrade, across the system based on maximum power dissipation from all the cards. An objective might be to maintain the microprocessor heat sinks such that the CPU cores are at a specified temperature of 85 to 90 degrees centigrade. For a system having 10 cards having a power dissipation of 250 W each (for a total of 2.5 kW), it is believed that a flow rate of 0.150 m3/s (312 CFM) at about 200 Pa (0.8 in of water) of pressure drop is required. This pressure drop is very high, and is hard to maintain with the axial fans that are typically used in this context.
It will be appreciated that there is a need for a system and an apparatus for effectively cooling the heat dissipating components of a rack-based electronic system having a high density of heat-generating components. There is also a need for a system and apparatus having good characteristics in protecting components from shock, vibration and/or dust, and particularly one with a minimum of size and cost. Preferred embodiments of the present invention satisfy some or all of these needs, and provide further related advantages.